breaking the cycle of surface scaling on concrete flatwork · 1. understand the causes of surface...

Breaking the Cycle of Surface Scaling on Concrete FlatworkMatthew R. Sherman, P.E., F-ACI, F-ICRIR. Brett Holland, Ph.D., P.E., S.E. 22 October 2019

2

Learning Objectives1. Understand the causes of surface scaling on concrete

formwork.2. Identify factors leading to the recent onset of scaling issues,

including weather, construction practices, materials, competition, pricing, specifications, expectations, and supervision.

3. Recognize common strategies for avoiding concrete surface scaling.

4. Discuss ideas for avoiding and mitigating these effects.

3

Our recent experience…

4

Our Experience – Common Causes• Low entrained air content• Overworked surface• Poor curing• Poor specification• Early exposure• Deicing salt attack• A combination of the above (over 50% of the time)

5

Key things to remember today…• Everyone is annoyed, exasperated, and frustrated by this

problem.• We are going to ask you to relax your preconceptions:

• There are many concrete “diseases” with scaling symptoms – it has to be properly diagnosed and responsibility is different for each

• If there are “n” people in the room, each knows the “n-1” people responsible for the problem

• Spoiler Alert: There isn’t one thing happening and there isn’t one solution.

5

6

Bottom LineNone of us wants: • Rework• Money-loser projects• Delays to users• Added maintenance • Claims/disputes

7

What is scaling?

8

Scaling – What is it & why does it happen ?

8

PCA: “…caused by the expansion of water due to freezing and thawing cycles and the use of deicing chemicals; however properly specified, produced, finished, and cured quality concrete need not suffer this type of deterioration.”

9

Scaling at the microscopic level…

10

Mechanism…

10

11

Mechanism…

11

Cl-

Mg++

Cl-

Mg++

Cl-

Na+

Cl-

Na+

Cl-

Mg++

13

Two types of scaling• Exposure of young or “green” concrete to aggressive

conditions

• Freezing and thawing of non-durable concrete

14

Early-age exposure scaling is caused by…• Freezing of concrete that still contains placement water

• Effects of salts on young (weak) concrete

15

Non-durable concrete is caused by…• Lack of

entrained air

16


entrained air

• High w/cm

17


entrained air

• High w/cm• Low

hydration

18


entrained air

• High w/cm• Low

hydration• Low

strength

19

But don’t count out the environment…• Early freezes

• Aggressive chemicals

• Saturation

20

So what is going on?

21

So what is going on?• Many things are in play• There are many mechanisms

that all manifest as scaling• There are lots of different

people and steps involved• You have to investigate to

find out which one or who

• Likely contributors:• Exposure• Finishing• Deicing salts• Materials• Specifications• Curing• Economics

22

Framing the discussion…• We have

always had concrete

23


always had concrete

• We have always had winter

24


always had concrete


Concrete Exposure

25


always had concrete


• It worked pretty well in the past

26

Exposure…

• Later construction seasons

27

Exposure…


28

Late season requirements…• 500 psi required before FIRST freeze (more for harsher exposure)

• Concrete has to lose excess moisture before it freezes• 3,500 psi before repeated cycles• 4,500 psi before repeated cycles with deicers (Per ACI)• “Green” concrete is especially vulnerable to deicing salts• Protection recommended if the concrete is exposed early in life

28

29

Late season requirements…• We are seeing late placements, with distress clearly correlating to the placement season

• But…economic advantages to late season placements means we have to balance risk and reward

29

30

Exposure…


• Changes in the weather

31

Exposure…


• Changes in the weather

32

Exposure…

• Number of cycles?

33

Exposure…


• Severity of winter?

34

Exposure…



35

Exposure…



36

Exposure…



• Timing of first freeze?

37

Finishing…• The finisher is

usually the first one blamed…

• Sometimes it is warranted…

• We aren’t “finishing it the same way we always have…”

38

Finishing…• The finisher is

usually the first one blamed…

• Sometimes it is warranted…

• We aren’t “finishing it the same way we always have…”

39

Finishing…• Isn’t being done “the same

way we always have…”• Changes in concrete• Labor changes• Economics

• And sometimes it isn’t being done correctly…

40




• Can remove air from the near-surface.

41




• Can remove air from the near-surface.

• Can trap bleed water.

42

Finishing…what about certification?• ACI Flatwork finisher certification

exists• It doesn’t exactly cover sidewalks• Not a bad idea, but…• Has associated costs

• Actual time and money• Potential for “beta error”

• Only means you know how to do it “correctly”, not that you will

• Needs incentives, such as pay factors, and oversight to make it worthwhile

PCA trowel photo – used with permission

43

De-icing…• De-icing is known to exacerbate the problem

• De-icing has changed over the years• More emphasis on clear paths of travel• New materials • Pre-treatment of salt and pre-application of brines

44

De-icing…• De-icing is known to

exacerbate the problem• De-icing has changed

over the years• More emphasis on

clear paths of travel• New materials • Pre-treatment of salt

and pre-application of brines

45

De-icing…• Is sometimes clearly

associated with damage patterns, sometimes not…

• Note that concrete should be able to survive deicing salts, but our testing hasn’t evolved to address changes in de-icing technology…

46

De-icing – when can I first apply them?• Sometime between now and never.• Industry does not have a definitive answer.

• “1 to 3 months”• “1 year”• After the first freeze-thaw cycle.• “After a period of drying”

• Most of these are UNREALISTIC• Our recommendation:

• Best bet is to not de-ice for the first winter• More realistically, do not de-ice until 2 to 3

months • Protect the concrete with a silane or siloxane

sealer• Read the bag for application rates

46

48

Materials…• Changes in

cements• Changes in

admixtures• Changes in

pozzolans

49

Materials…• Changes in

cements• Changes in

admixtures• Changes in

pozzolans

50

Materials• Cements are finer• SCMs are common, we are

even seeing blends of three• SCM sources are in flux• These change

• Finishing “feel”• Timing of finishing • Bleed• Strength gain

51

Material effects include…• Air-entrainers are different

now; typically see synthetics• Interactions with other

admixtures, particularly polycarboxylates

• Sensitivity to process

• These changes don’t make the concrete “bad”, they make it different

PCA photograph – used with permission

52

Specifications…• Sidewalks are often

an afterthought• Not really ACI 318,

not really AASHTO…• Often unrealistic

curing• Haphazardly

enforced• Weak on testing• Haven’t been

updated to reflect current practice

53

Specifications…• Sidewalks are often

an afterthought• Not really ACI 318,

not really AASHTO…• Often unrealistic

curing• Haphazardly

enforced• Weak on testing• Haven’t been

updated to reflect current practice

54

Specifications…Strength

(psi) w/cm Air SCM Limits

ACI 201.2-01 4,000 0.45 4.5-7.5 [no mention]ACI 201.2-16 4,500 0.45* 4.5-7.5 Max. limits given

Mass. Building Code 3,500 - 4.5-7.5 [no mention]

AASHTO(LRFD Bridge) 4,000 0.45 4.5-7.5 [no mention]

Typical DOT 4,000 - 5.0-6.0 Min./Max limits givenACI 318-95 4,500 0.45 4.5-7.5 Max. limits givenACI 318-14 5,000 0.40 4.5-7.5 Max. limits given

City of Boston 4,000 - 5.5-7.5 None allowed

• All for wet, with deicer exposure

55

Specifications…To think about• Recommendations are

based on prior testing and have evolved; have your specifications kept up?

• Deicing technology has changed; does what worked before work now?

Concerns• Finishability challenges• Hot weather protection• Curing becomes more

important• Cost: labor and materials

56

Specifications• Check your testing

requirements and frequency• Owners• Contractors• Producers

• Enforce your testing requirements to level the field

• Testing is only as good as the tester

• But remember, the in-situ concrete isn’t tested

57

Curing…• Is critical to

performance• Provides for

hydration and reaction of the cement in the near-surface concrete

58

Curing…• Is critical to

performance• Provides for

hydration and reaction of the cement in the near-surface concrete

59

Curing• Absolutely necessary for good

concrete• Absolutely impractical for

miles of sidewalk when you have to keep businesses open

• Curing compounds are a mixed bag

• Help early• Can interfere with later

coatings• New cure/seals look promising

60

Economics…• Are the driver of

the entire system• Critical to staying

competitive in the market

• Affect finishing, curing, materials, exposure, time to first exposure…

61

Economics…• Are the driver of

the entire system• Critical to staying

competitive in the market

• Affect finishing, curing, materials, exposure, time to first exposure…

62

Maybe a combination…• We have

always had concrete



63

Maybe a combination…• We have

always had concrete



Maybe multiple things changed…

64

Let’s collect our thoughts

65

So far, we have learned…• There is a lot going on here• It is a systems-level problem,

not an item-level problem• As a system-level problem,

there is a real question of system “robustness”

• Different types of problems manifest in a similar way, making them hard to tell apart

66

Likely contributors…• Exposure• Finishing• Deicing salts• Materials• Specifications• Curing• Economics

67

So how do we move forward?

68

Let’s look at some options…

• Group by when they get implemented

• Realize that different ones are appropriate for different projects – you need to balance cost, risk, and reward• Late-season placement at the entrance to a signature building?• June placement at the dumpster pad?

69

Before construction…• Specifications – materials, testing, curing, sealers…

• Make sure that all are bidding to the same expectations and requirements – this may take some education and time

• Probably want 0.40 w/cm; 5,000 psi strength, 5.0-8.0% air• Probably an acrylic/silane-based cure and seal• Probably want to test the concrete often – maybe even core• Pre-qualify concretes

• Planning – timing, crews, suppliers, expectations…• Start testing mixes, maybe with updated tests that reflect today’s practices and

deicers• Payment factors – to reinforce & reward doing it the right way, all the

time

70

During construction…• Use proper techniques and best practices

• Incent and motivate the workers• Certification of finishers – maybe, but that won’t solve problem if

finishers don’t want it to, and runs risk to the labor force• Inspection, testing, and monitoring

• Hold everyone to the same standards• Testing for what matters is not so straightforward• “Keeps honest people honest” and levels the playing field

• Protect and cure• Prevent early exposure• Give the concrete a chance to succeed

71

Avoiding scaling – after construction…• Control the exposure

• Choose your deicer carefully• Apply as recommended, and to minimum extent required• Consider alternates to early salting• Remove excess and rinse as often as you can

• Consider sealers• Provide additional margin, especially for concrete cast late in

season• Benefits likely outweigh the costs

72

Wrap up…

73

A multi-phase issue with many contributors…• Everyone owns a bit, and everyone can help improve the situation• Requires a cooperative response• Adjust the way we think about it – they are big $, and big impact• Need to recognize the economics of the projects

• Maintain an even field with uniform expectations, testing, and enforcement

• Incent all parties – share risk and reward

• We can be successful if we all do our part!

Questions?This concludes The American Institute of Architects

Continuing Education Systems Course

www.sgh.com

Matthew [email protected]

Brett [email protected]

78

SECTION BREAK

Option 11. Keep this photo as your background image.2. Delete this text box when finished.

Option 21. Insert new, full-bleed photo to replace this background image. 2. Send new image to background.

1. Right click on photo.2. Hover over “Send to Back” to open sub-menu. 3. Click on “Send to Back” in sub-menu.

3. Delete this text box when finished.

79

ACI 201, ACI 318 and AASHTO – Where does concrete flat work fit?

80

Durable concrete is…Knowing your Exposure Class (ACI 201 and 318, which are the same)

80

ACI 201.2R-16 Exposure Classes

81

Durable concrete is…andDesigning your concrete mixture to meet the freeze-thaw durability requirements of ACI 201

82

Durable concrete is considering the enhanced freeze-thaw durability requirements of ACI 201

83

Durable concrete is considering AASHTO Requirements

• Limits w/cm to 0.45

• Requires providing a minimum 30 day drying period after curing before allowing the use of deicers.

84

Why Air-Entrainment Important?

Acts at Air-Water Interface

PCA

§ AEA’s are surfactants.§ Non-polar chain molecule,

with a -ve charge at one end§ Cause water to foam during

mixing

Molecular orientation

lowers surface tension at air/water interface

§ Entrained air voids (~0.05-1mm) remain stable throughout hydration and remain after setting

85

Characteristics of Frost Resistant Air Void System

• Spacing factor < 0.008 in (0.203 mm)

• Specific surface > 600 in2/in3 (24 mm2/mm3)

• Voids per linear inch: 1.5 - 2 times the percentage of air

ASTM C 457

86

Air Void Spacing & Volume• Spacing factor ( L): the maximum

distance of any point in the thecement paste from the periphery of an air void.

• Specific surface: the surface area of a quantity of air voids that have a total volume of 1 in3 (16 cm3).

L

L

Courtesy of M. Thomas

87

Air-Entraining Admixtures (AEAs)• 4 Categories:

1. Wood Derived Products: Vinsol® resin, Tall oil, Wood rosin (Less available)

2. Synthetic Materials: Alky-aryl sulfonates and sulfates (Now most common)

3. Vegetable Acids: Coconut fatty acids, Alkanolamine salt4. Miscellaneous: Alkali/alkanolamine acid salts, Animal tallows

• Must pass ASTM C 260

88

Measuring Air in Hardened Concrete

• ASTM C 457 provides 2 methods:A: Linear Traverse Method

B: Point Count Method

PCA

89

Good Air

Low Air

Low Air in the Surface Region (Overfinishing)

90

What’s Different Now?

91

Batching and Transporting Durable Concrete• Batch to meet the ACI design requirements for air and

w/cm

• Know how your AEA’s perform • Slump;

<3-in. (75mm),>6-in. (150mm)

• Temperature• Quality Control Testing (more on this later)

92

Placing Finishing and Curing Durable Concrete•Placement/ Pumping•Consolidation

•Finishing

•Curing

PCA photo

93

Quality Control Concrete materials

Cement • Admixture dosage based on cement content• Higher fineness requires higher dosage• Alkalies increase air entrainment

Supplementary Cementitious Materials• Fly ash, silica fume may increase dosages 2 – 6

times• Be careful for unburned carbon in fly ash

Aggregates• Increased sand content helps air entrainment• Ultra fines (< #200) harmful

Admixtures• Compatibility should be examined• Added separately to mixtures• Some superplasticizers affect spacing factor

QC Testing of Materials and Concrete; Submittals

94

Measuring and Monitoring Air Content in the Field

ASTM C 231 Pressure Method

PCA photosASTM C 173

Rollometer Method ASTM C 138 Gravimetric

Method

Measure total air volume in a sample compacted in a standard way.Cannot distinguish between entrained and entrapped air.Does not consider effects placing/finishing operations.

PCA photos

95

Avoid early-age exposure by…• Avoid placing flatwork after September

• It can be done, but you have to understand the risks

96

What can I do about freeze-thaw scaling?• Confirm the cause and determine the extent

• Good investigation, including a hands-on survey; use a chain drag and hammer

• More survey = fewer surprises = more certainty • Remove samples and test (petrography, NOT strength)

• Determine the response• How deeply is the concrete affected?• How widely is the concrete affected?• How much do I have to spend?• How do I combine these needs?• Do I just fix what is bad today or slow/stop it in the future?

96

97

Conclusions• We can improve the odds and achieve scaling

resistant flatwork

• We need to consider life-cycle cost

• If we do not want to lose the market to asphalt we need a cooperative response to minimize this problem

Scaling Notes Conclusions• Freezing scaling is caused by water,

and made worse by de-icing salts• Nothing protects saturated

concrete, so let it dry• Use a sealer or membrane• Avoid placing late in season, if you

do, use a sealer!!• Repair is hard – it will never be the

same• Use a durable concrete

breaking the cycle of surface scaling on concrete flatwork · 1. understand the causes of surface...

Documents